A control system disclosed in Patent Document 1 sets a target value of a maximum lift of an engine valve based on an engine operating state in such a manner as to improve the fuel efficiency and output. The control system performs a feedback control so that the actual value of the maximum lift becomes equal to the target value. Typically, the control system is configured as described below.
The control system includes an actuator changing the maximum lift and an encoder outputting a pulse signal based on operation of the actuator. A counter circuit detects the change history of the maximum lift by selectively increasing and decreasing a count value based on the pulse signal output by the encoder. The counter circuit is powered by a backup power source. When the power supply from the backup power source is suspended, for example, when the internal combustion engine stops operating, the count value is reset to 0 regardless of how long such suspension of the power supply lasts, and the change history is cleared.
After the engine is started, a microcomputer selectively charges and discharges a memory cell of a volatile memory of the microcomputer through the backup power source, thereby storing the count value of the count circuit, or the change history of the maximum lift from the initial value at the time when the engine was started. When the engine is stopped, the final value of the maximum lift is stored in a rewritable nonvolatile memory and used as the initial value of the maximum lift after the engine is restarted. The microcomputer calculates the actual value of the maximum lift based on the change history and the initial value of the maximum lift stored in the volatile memory. The microcomputer changes the maximum lift of the engine valve through the actuator in such a manner as to reduce the difference between the actual value and a target value that is set based on the operating state of the engine.
However, vibration of the vehicle body or the engine may cause a contact failure in a feeder circuit between the backup power source and the microcomputer. Specifically, temporary suspension of the power supply from the backup power source, that is, a temporary blackout, may occur. Despite the temporary power blackout, the data of the change history stored in the volatile memory remains until a certain period of time elapses after the suspension of the power supply. The remaining data is usable after the power supply is restored if the content of the data remains unchanged. However, since the state of the power supply becomes unstable before and after the temporary power blackout, the content of the data may change. Also, when vibration of the vehicle body or the engine occurs successively, temporary power blackout may reoccur in a short period of time.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-201117